1. Field
The present invention generally relates to communication systems. More particularly, the invention relates to backoff control in communication systems that use access probes.
2. Background
Wireless communication systems have developed through various generations, including a first-generation analog wireless phone service (1G), a second-generation (2G) digital wireless phone service (including interim 2.5G and 2.75G networks) and a third-generation (3G) high speed data/Internet-capable wireless service. There are presently many different types of wireless communication systems in use, including Cellular and Personal Communications Service (PCS) systems. Examples of known cellular systems include the cellular Analog Advanced Mobile Phone System (AMPS), and digital cellular systems based on Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), the Global System for Mobile access (GSM) variation of TDMA, and newer hybrid digital communication systems using both TDMA and CDMA technologies.
The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association/Electronic Industries Association in TIA/EIA/IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” referred to herein as IS-95. Combined AMPS & CDMA systems are described in TIA/EIA Standard IS-98. Other communications systems are described in the IMT-2000/UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), CDMA2000 (such as CDMA2000 1xRTT, “1x”, and 1xEV-DO standards, “1XEV”, for example) or TD-SCDMA.
In wireless communication systems mobile terminals or access terminals receive signals from fixed position base stations (also referred to as cell sites or cells) that support communication links or service within particular geographic regions adjacent to or surrounding the base stations. In order to aid in providing coverage, each cell is often sub-divided into multiple sectors, each corresponding to a smaller service area or geographic region. An array or series of base stations placed adjacent to each other form a communication system capable of servicing a number of system users, over a larger region.
Typically, each mobile terminal monitors a control channel that can be used to exchange messages between the mobile terminal and the base station. The control channel is used to transmit system/overhead messages, whereas traffic channels are typically used for substantive communication (e.g., voice and data) to and from the mobile terminal. For example, the control channel can be used to establish traffic channels, control power levels, and the like, as is known in the art. Generally, there are two types of power control for the reverse link, open-loop and closed-loop power control. The open-loop power control typically occurs prior to the mobile terminal establishing contact with a base station. The closed-loop control occurs after the mobile and the base station are in communication and the base station can measure the received power levels and feedback power level adjustments to the mobile terminal.
In the open loop condition, the reverse link power for an initial communication signal (e.g., access probe) from the mobile terminal to the base station can be determined by monitoring specialized signals from a base station or access point. For example, in CDMA systems a pilot signal can be use to estimate the channel condition and then determine a power estimate for transmitting back to the base station. The accuracy of the channel conditions and power estimation can greatly impact performance of the system, particularly in terms of latency of the system. For example, 1x and 1XEV systems will transmit an access probe at a first power level based on a power control algorithm. If the first access attempt does not succeed, then the probe is resent at increasingly higher power levels, until it is successful or the power level maximum is reached.
The existing open-loop power control algorithm used to transmit access probes over the Access Channel in CDMA2000 1X-A and 1xEVDO networks tend to be prone to inaccuracies and can result in underestimation of transmit power for access probes. This leads to an increased loss rate of access probes over the Access Channel, particularly on the first access attempt. Accordingly, errors in the determination of the power level for the first transmission can lead to a high rate of unsuccessful first access attempts, which can cause increased system latency as the probes are resent. By limiting retransmissions of the access probes, the latency incurred by access probes can be reduced.
Another cause of failed access attempts is collision between access probes. Collisions occur when more than one mobile terminal attempts to send an access probe on the same Access Channel in the same sector. Because of the interference cause by the competing signals, the base station may not successfully receive the access probes. Accordingly, collisions are another factor that can impact the latency of a communication system. To address the problem of collisions, some conventional systems will generate a random backoff time to prevent collisions on subsequent retransmissions. However, conventional systems do not address potential collisions with the first access probe. Accordingly, system latency can also be impacted by collisions on the initial access probe attempt.